The present invention relates to an electrophotographic magnetic carrier and a process for producing the magnetic carrier, and more particularly, to an electrophotographic magnetic carrier not only having a high saturation magnetization and a freely controllable charge amount, but also being free from falling-off or separation of magnetic fine particles from a core particle, for example, in case where a granulated particle comprising the magnetic fine particles is used as a core particle.
As is well known in the conventional electrophotographic processes, there has been adopted an image-developing method comprising using a photosensitive conductor formed of a photoconductive material such as selenium, OPC (organic photo-semiconductor) or amorphous silicone, forming an electrostatic latent image on the photosensitive conductor by various methods and electrostatically attaching to the latent image a toner charged to a polarity reverse to that of the latent image.
In such an image-developing method, there have been used particles called "a carrier" which is brought into frictional contact with a toner to impart an adequate amount of positive or negative charge to the toner. The thus charged toner is transported through a developing sleeve using a magnetic force exerted by a magnet accommodated in the developing sleeve, to a developing zone adjacent to a surface of the photosensitive conductor where the latent image is formed.
In recent years, the electrophotographic techniques have been widely applied to copying machines, printers or the like. In these applications, it has been required to accurately reproduce fine lines, small characters, photographs, color images or the like. Further, it have also been required to achieve a high-quality or high-grade image, a high-speed or continuous operation or the like. These demands are expected to be more and more increased in the future.
As conventional carriers, there have been used magnetic particles such as iron powder (a mechanically crushed iron powder, an electrolytic iron powder, a reduced iron powder, a heat-treated iron powder, a sintered iron powder or the like), ferrite particles (Mn ferrite particles, Li--Mn ferrite particles, Ni--Zn ferrite particles, Mn--Zn ferrite particles, Cu--Zn ferrite particles or the like) or magnetite particles or the like. However, any of these particles exert a large stress against toner due to an impact force therebetween when both are mixed and agitated together in a developing device, so that the developer suffers from deterioration in its durability during a long-term use.
In addition, irregularities on surfaces of magnetic particles causes the same problem concerning the durability of developer as mentioned above, especially when toner is deposited into concave portions thereof.
Further, in some methods of producing magnetic particles, there arises such an disadvantage that a large amount of fine particles having as small a diameter as not more than 1 .mu.m are present therein. The fine particles tend to be fallen-off or separated from the surface of the magnetic particle which, for example, comprises magnetic fine particles, thereby causing such a problem that when the magnetic particles are mixed with a colored toner, the color tone of the toner is deteriorated. Especially, in the case of yellow-colored toner, the above-mentioned problem becomes more remarkable.
In order to solve these problems, there have been proposed resin-coated magnetic particles obtained by coating as a core particle a magnetic particle of a granulated particle comprising magnetic fine particles or iron powder (iron particle) with an insulating resin as a carrier. However, in the case where the granulated particle comprising the magnetic fine particles is coated with a resin in a large thickness to inhibit falling-off or separation of magnetic fine particles from the core particle, the volume resistivity of the carrier itself becomes too large, thereby causing such a problem that images having a deteriorated quality are produced, e.g., images showing too sharp edges or conversely solid images having too low toner density.
Further, when the core particle is coated with a resin, irregularities on the surface of each core particle are reflected on the surface of resin layer formed thereon, so that there is caused a so-called "spent" phenomenon that toner is deposited into concave portions thereof during a long-term operation. Thus, in the case of the resin-coated magnetic particles, the problem concerning the durability of developer still remains unsolved. Furthermore, the conventional resin-coated magnetic particles are deteriorated in adhesion between the core particle and the coating resin, thereby causing such a problem that the resin-coating layer is peeled-off or separated from the core particle during a long-term use.
In order to solve the former problem concerning the high volume resistivity, in Japanese Patent Applications Laid-open (KOKAI) Nos. 2-120750(1990) and 3-72372(1991), it has been proposed to incorporate a conductive material such as carbon black or metal oxides into the resin-coating layer. However, there still exist problems that the content of the conductive material in the resin-coating layer is insufficient to reduce the volume resistivity, and that the adhesion between the core particle and the resin-coating layer is unsatisfactory.
On the other hand, in order to solve the latter problem concerning the adhesion between the core particle and the resin-coating layer, in Japanese Patent Applications Laid-open (KOKAI) Nos. 64-29857(1989) and 62-121463(1987), there has been proposed methods of preliminarily surface-treating the core particle with a coupling agent such as a Si-based coupling agent, a Ti-based coupling agent or an Al-based coupling agent. In the case of using the method described in Japanese Patent Application Laid-open (KOKAI) No. 64-29857(1989), the resin-coating layer is constituted by thermoplastic resin polymer particles, thereby causing such a problem that when the resin-coated particles are mixed with toner in a developing device, there is caused a fusion therebetween. In the case of using the method described in Japanese Patent Application Laid-open (KOKAI) No. 62-121463(1987), no additives are contained in the resin-coating layer, thereby causing the above-mentioned problem concerning the electrical resistivity.
Also, in order to essentially solve these problems, there have been proposed a so-called resin carrier in which magnetic fine particles having a diameter of about 0.1 to about 2 .mu.m are dispersed in an insulating resin.
Since the resin carrier has a light weight, the stress exerted against toner when agitated therewith in a developing device is small, so that a long life of developer can be assured.
However, since the resin carrier comprises about 30 to about 50% by weight of the insulating resin and the magnetic fine particles, the saturation magnetization of the carrier becomes low, thereby causing a so-called carrier adhesion, i.e., such a phenomenon that the carrier scattered from the magnet roll of the developing device during use, adheres to the surface of the photosensitive conductor. As a result, there arises a problem that voids are formed in obtained images. Therefore, it is required to recover the carrier from the surface of the photosensitive conductor or replenish the carrier in the developing device.
Especially in recent years, the toner has been required to have much smaller particle size, specifically in order to achieve a high image quality. For this reason, it has also been required to correspondingly reduce a particle size of the carrier itself. This results in decrease of magnetization per one carrier particle, so that the above-mentioned problems tend to be frequently caused.
In addition, the printing speed of recent copying machines or printers becomes considerably higher as compared to those of conventional ones. Specifically, in order to increase the printing speed, it is required to increase the developing speed. As a result, it is necessary to provide such a carrier which can be firmly retained on the developing sleeve even when the sleeve is rotated at a high speed. That is, the higher magnetization of the carrier is required.
As a result of the present inventors' earnest studies for solving the above-mentioned problems, it has been found that by treating the surface of a magnetic particle as a core particle with a surface-treating agent having an amino group to form a coating layer comprising surface-treating agent having an amino group, on surface of the magnetic particle, and reacting phenols with aldehydes in an aqueous solvent containing the above-treated magnetic particles, inorganic fine particles subjected to a pre-treatment for imparting a lipophilic property thereto in the presence of a basic catalyst to form an outer layer comprising inorganic fine particles and a cured phenol resin on a surface of the surface-treating agent layer, the produced composite particles are useful as an electrophotographic magnetic carrier. The present invention has been attained on the basis of this finding.